The present invention relates to a system and method for monitoring the integrity of a satellite transmission. More specifically, the present invention relates to a system and method for verifying the integrity of Global Position System (GPS) satellite transmissions.
The Global Positioning System (GPS) consists of 24 earth-orbiting satellites. The GPS satellites broadcast a navigation message via a radio frequency (RF) signal. This signal allows any individual with a GPS receiver to process the GPS signals and determine his or her precise longitude, latitude, altitude, velocity and time anywhere in the world.
Although Global Positioning System (GPS) provides very accurate position and time information, there are times when GPS satellite system malfunctions can introduce errors into the GPS signal transmitted from the GPS satellite. When this occurs, the GPS receivers will not be able to accurately determine position and/or time. Past data has shown that the GPS signal has typically malfunctioned on the average of around 45 minutes a year. When the GPS satellite system is functioning properly and producing accurate GPS data, the GPS data is described as having xe2x80x9cintegrity.xe2x80x9d
GPS errors can be caused by a number of conditions. If one of the GPS satellite transmitter elements or other satellite components fail, the GPS signal waveform can become corrupted. For example, an output amplifier in the GPS satellite may start to malfunction and thereby corrupt the transmitted signal. Another source of error is the failure of the satellite""s atomic clock. If a clock failure occurs, the satellite will transmit incorrect time data and introduce error into the computed position information. Another potential error is the transmission of erroneous correction data from GPS ground stations to GPS satellites. GPS ground stations uplink correction data to the GPS satellites every 24 hours. If a ground station sends the wrong correction data, then the GPS satellites will produce inaccurate or erroneous output signals. As an example of this type of error, a ground station could mistakenly send correction data for Tuesday when it was supposed to send correction data for Wednesday.
Errors in GPS signals can lead to severe safety issues or inefficient operation for many systems that use GPS signals such as aircraft systems, transportation systems, weapon systems and so forth. New aircraft navigation systems are being developed which rely on GPS signals for navigation. Errors in the received GPS signal could lead to mid-air crashes. The Federal Aviation Administration (FAA) has a goal of having no more than a 2xc3x9710xe2x88x928 probability of error in the GPS signal without an alert that the signal is hazardous or misleading. With the current GPS system, the probability of error in the GPS signal is on the order of 10xe2x88x924 per satellite per hour or even higher. Thus, the current GPS satellite could produce a probability of error ten thousand times higher than the FAA""s desired goal.
Other proposed systems which utilize GPS signals include intelligent highway systems. These intelligent highway systems use GPS signals to manage traffic by providing autonavigation for the automobiles on the freeways. Similar systems have been proposed for trains. Thus, it will be a very important safety issue for these systems to ensure the integrity of the received GPS signals.
Currently, the Global Positioning System (GPS) system does not have any form of integrity monitoring as part of the system. A system known as the xe2x80x9cWide Area Augmentation Systemxe2x80x9d (WAAS) is currently being designed and developed to provide integrity monitoring of GPS. The WAAS will use a series of new ground stations at known locations all over the world. Each ground station will include a satellite antenna which receives GPS signals from the in-view GPS satellites. Each ground station will use these GPS signals to calculate its own position. By comparing the calculated position with the known position of the ground system, the accuracy and the integrity of the GPS signal can be determined.
If the calculated position is different from the known position, a correction message is generated by the ground station. The ground station transmits the correction message to an independent messaging system such as a geosynchronous satellite. This geosynchronous messaging satellite then broadcasts the correction message to all GPS users in the region. The GPS users then use the correction message to correct the GPS data received from the GPS satellites. Alternatively, the geostationary messaging satellite can transmit an integrity message to all GPS users in the region, informing the users of a potential satellite malfunction. GPS users can thereby be informed that they should not rely on the GPS signals being received. Alternatively, the ground system could send the integrity message to a mission control system which sends a message to the GPS satellites to correct the erroneous data or to cease transmitting all GPS navigation data.
This planned WAAS integrity monitoring system will require an enormous cost including the cost of building the new WAAS ground stations, procuring the new geostationary messaging satellites, and the costs of maintaining and operating the ground stations. Estimated costs for the development and implementation of WAAS are greater than 2 billion dollars. Moreover, the WAAS may not be able to signal a problem with GPS integrity with sufficient speed. Systems using GPS frequently need to know of a change in GPS signal integrity in times less than 1 sec after a malfunction or error occurs.
What is needed is a system that can provide a high level of confidence in GPS integrity without the enormous cost and complexity associated with the WAAS. What is also needed is a system that can correct GPS errors or alert GPS users to a loss of integrity with sufficient speed to satisfy safety concerns and regulatory standards. Lastly, what is needed is a system that can be used either on its own or in conjunction with a WAAS-type monitoring system to provide a high level of confidence in GPS integrity for use in navigation systems, aircraft landing systems, transportation systems, weapon systems, and many other types of systems to provide increased safety and efficiency.
The present invention is a system for providing GPS users with a high level of confidence in the integrity and accuracy of received GPS signals. The present invention could also potentially be used for satellite systems or space vehicles other than GPS.
The system of the present invention allows the GPS satellite constellation to automatically locate itself with respect to a fixed beacon on the earth. The beacon is located at a fixed point on the earth at a known location. The beacon emits a coded signal pulse having a precise RF waveform that is received by all GPS satellites in view of the beacon""s location. The RF signal pulse emitted by the beacon is detected by sensors on the GPS satellites. The RF signal pulse is decoded and processed onboard each GPS satellite.
Each GPS satellite calculates beacon position data ""such as the distance between the GPS satellite and the beacon. Each GPS satellite then transmits the beacon position data to the other in-view GPS satellites via inter-satellite links. The beacon position data allows each GPS satellite to calculate the relative position of the beacon. Once each GPS satellite determines its position relative to beacon, the GPS satellite calculates its own coordinates in space, since the earth coordinates of the beacon are known. Once the position of the GPS satellite is calculated, this calculated position is then compared with other satellite position data to verify the integrity of the GPS system. The other satellite position data can include 1) ephemeris data received from a ground station, and/or 2) Autonav position data. The position of the GPS satellite from either or both of these sources can be compared to the satellite position determination based on the beacon registration. If the positions match within a certain error, then the GPS satellite can verify its own integrity.
After a GPS satellite verifies the accuracy and/or integrity of its own operation, the GPS satellite transmits an integrity message to all GPS users in view of the satellite. The integrity message can alert GPS users to a loss of integrity or accuracy in the GPS signals. Alternatively, the integrity message can contain information to correct errors in the GPS signals. The integrity message can be incorporated into the existing GPS navigation message transmitted by GPS satellites, or the integrity message can be transmitted over a separate channel such as the planned L5 band channel.
GPS receivers will receive and decode the integrity messages transmitted by individual GPS satellites. The GPS receivers will then be able to determine whether or not the GPS signals being received have integrity. If the integrity message contains correction data, the GPS receivers can use the correction data to correct the GPS signals. The GPS users will thus be provided with very high confidence in the integrity, accuracy, and reliability of the GPS position and time data. This confidence will enable many new applications to be adopted by the civil community and general public.
Because the system of the present invention allows the GPS satellites themselves to verify their own integrity and/or accuracy, the system eliminates the enormous cost and complexity associated with proposed ground-based GPS integrity monitoring systems like the WAAS. The system of the present invention also provides faster response times and more robust operation than proposed ground based monitoring systems. Additionally, because the GPS satellites themselves report their integrity to GPS users, the need for an independent messaging system is eliminated. As an option, the system could be used in conjunction with a ground-based monitoring system like WAAS, to provide the highest degree of integrity and the lowest probability of GPS error.